1. Field of the Disclosure
The present invention relates generally to image sensors. More specifically, examples of the present invention are related to circuits that readout image data from image sensor pixel cells.
2. Background
Image sensors have become ubiquitous. They are widely used in digital cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular complementary metal-oxide-semiconductor (CMOS) image sensors has continued to advance at great pace. Furthermore, the increasing demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
The demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright light conditions are becoming more difficult to achieve as pixel cells become smaller. This performance capability is generally referred to as having high dynamic range (HDR). In conventional image capture devices with small photosensitive devices, the pixel cells often require multiple successive samples or exposures of the photodiodes with long and short integration times to achieve HDR.
In a conventional CMOS pixel cell, image charge is transferred from a photosensitive device (e.g., a photodiode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. A challenge with this approach is that each readout of the conventional pixel cell is destructive. In particular, the charges in a photodiode disappear after each readout, which reduces light sensitivity compared to pixel cells that can accumulate light during an entire frame time.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.